The present invention relates generally to testing and evaluation of the data recovery processing system in magnetic data recording and storage systems. In particular, it relates to measuring the bit error rate arising during the readback process as a function of data window size.
In the data recovery process in the typical digital magnetic storage system, the interval during which a readback pulse representing a bit of data is detected is referred to as the "data window". In the ideal system, having a noiseless channel and no intersymbol interference, there is no "bitshift", i.e. all pulses occur in center of the data window. In real systems, however, particularly in high density storage devices, readback pulses representing data bits are shifted with respect to center of the data window as a result of density of recorded data and the code patterns of such data in the data signal. See, for example, an article entitled, "Effect of Bitshift Distribution on Error Rate in Magnetic Recording", IEEE TRANSACTIONS ON MAGNETICS, Vol. MAG-15, No. 3, May 1979 by Eric R. Katz and Thomas G. Campbell.
In addition to interaction-induced bitshift, readback pulses are also subject to additional noise-induced bitshift. If pulses are shifted enough to fall outside the data window, they are counted as errors. Such information may be used to predict the right error rate in lieu of a direct measure of such error rate which generally requires considerable time.
A recording system may be evaluated directly, however, by measuring the error rate as a function of data window size. Historically, such a measurement has been tedious, time-consuming and costly, and the accuracy is typically unsatisfactory. In one system, described in U.S. Pat. No. 4,394,695, two methods for deriving error rate are described. In one arrangement, raw data is applied to a window generator which generates a window of predetermined width at the data clocking rate. The raw data is also applied to delay element which delays the data predetermined amounts with respect to the front edge of the data window. By selectively delaying the data signal, the data window is effectively narrowed. The narrowed window causes some data bits to fall outside of the window and provide a measure of error rate. Alternatively, the patent describes a system for applying data directly to a plurality of window generators, each of which generates a window of different width, for making comparative measurements of error rate with respect to gradually decreasing window widths.